1. Field of the Invention
The present invention relates to a filter switching circuit for selecting one of a plurality of bandpass filters having different frequency bands that allow signals to pass through them.
2. Description of the Related Art
When using television signals on many channels arranged in a broad frequency band, a television signal transmitter divides the television signals into a plurality of smaller bands for transmission in order to prevent disturbance from occurring due to a beat signal between television signals on each channel, harmonics, etc. Accordingly, a plurality of bandpass filters for transmitting the signals in the smaller bands are provided, and one of the bandpass filters is selected.
FIG. 2 shows a conventional filter switching circuit that selects one of a plurality of bandpass filters and connects the selected one between two amplifiers. A printed circuit board 51 has a first amplifier 52 and a second amplifier 53 thereon, with a distance provided therebetween. The first amplifier 52 is formed in an almost rectangular area having a longitudinal dimension Y1 and a lateral dimension X1, and the second amplifier 53 is formed in an almost rectangular area having a longitudinal dimension Y2 and a lateral dimension X2. The output end 52a of the first amplifier 52 and the input end 53a of the second amplifier 53 are opposed to each other.
In an area between the first amplifier 52 and the second amplifier 53, on a side (the lower side in FIG. 2) to a straight line (not shown) connecting the output end 52a and the input end 53a, a plurality of bandpass filters (called a xe2x80x9cbandpass filter 54-1xe2x80x9d to a xe2x80x9cbandpass filter 54-8xe2x80x9d from the top) (hereinafter referred to also as xe2x80x9cbandpass filters 54xe2x80x9d for denoting the entirety of the bandpass filters 54-1 to 54-8) are provided in parallel so as to be almost perpendicular to the straight line and to have equal distances to the output end 52a and the input end 53a. 
The bandpass filters 54 are designed so that eight bands obtained by dividing a frequency range (e.g., 50 MHz to 600 MHz) can pass through them. The bandpass filter 54-1 has the highest frequency passband (510 MHz to 600 MHz).
The bandpass filters 54-2 to the 54-7 have sequentially lower frequency passbands, and the bandpass filter 54-8 has the lowest frequency passband (50 MHz to 60 MHz).
Between the first amplifier 52 and the bandpass filters 54, a plurality of relays 55-1 to 55-7 (hereinafter referred to also as xe2x80x9crelays 55xe2x80x9d for denoting the entirety of the relays 55-1 to 55-7) respectively corresponding to the bandpass filters 54-1 to 54-7 are provided. Between the bandpass filters 54 and the second amplifier 53, a plurality of relays 56-1 to 56-7 (hereinafter referred to also xe2x80x9crelays 56xe2x80x9d for denoting the entirety of the relays 56-1 to 56-7) respectively corresponding to the bandpass filters 54-1 to 54-7 are provided.
The relays 55 and 56 consist of, for example, relays having identical structures. Each of the relays 55 and 56 includes a switching terminal a, two fixed terminals to which the switching terminal a selectively connects, that is, a first fixed terminal b and a second fixed terminal c.
Among the relays 55, only the switching terminal a of the relay 55-1 is connected in the smallest distance to the output end 52a. The other relays 55-2 to 55-7 are connected to the output end 52a in seven stages in cascade such that their switching terminals a are sequentially connected to their second fixed terminals c when these are connected to the output end 52a. 
Similarly, among the relays 56, only the switching terminal a of the relay 56-1 is connected in the smallest distance to the input end 53a. The other relays 56-2 to 56-7 are connected to the input end 53a in seven stages in cascade such that their switching terminals a are sequentially connected to their second fixed terminals c when these are connected to the input end 53a. 
The bandpass filter 54-1 having the highest frequency passband is connected between the first fixed terminals b of the relays 55-1 and 56-1, and the bandpass filter 54-2 having the second highest frequency passband is connected between the first fixed terminals b of the relays 55-2 and 56-2. Similarly, the bandpass filters 54-3 to 54-6, which have sequentially lower frequency passbands, are connected between the relays 55 and the relays 56. The bandpass filter 54-8 is connected between the second fixed terminals c of the relays 55-7 and 56-7.
The switching terminals a of pairs of corresponding relays, that is, a pair of the relays 55-1 and 56-1 to a pair of the relays 55-7 and 56-7 are switched in cooperation so as to be connected to their first fixed terminals b or their second fixed terminals c.
For example, the relay 55-1 and the corresponding relay 56-1 in the first stage are switched in cooperation, and the relay 55-2 and the corresponding relay 56-2 are switched in cooperation. Similarly, the other pairs of relays are switched.
In the above structure, a bandpass filter having a higher frequency passband is connected by a smaller number of relays to the output end 52a and the input end 53a in the smallest distance. Therefore, also in a case in which the frequency of a signal to be transmitted is high, signal loss and level deterioration are reduced. In addition, influence on characteristics of the relays 55 and 56, such as a cutoff frequency and transmission loss, are reduced.
xe2x80x9cNevertheless, in the above structure, the bandpass filter having a higher frequency passband is selected by a smaller number of relays. However, since the bandpass filters 54 and the relays 55 and 56 are provided on one side to the straight line connecting the output end 52a of the first amplifier 52 and the input end 53a of the second amplifier 53, a space B is formed on the other side between the first amplifier 52 and the second amplifier 53 is formed. Accordingly, the space-using efficiency of the printed circuit board 51 deteriorates, thus causing a problem in that the entire size of the printed circuit board 51 enlarges.xe2x80x9d
It is an object of the present invention to provide a switching circuit that selects a bandpass filter having a higher frequency passband by using a smaller number of relays and connects between two amplifiers in a distance as shortest as possible.
To this end, according to an aspect of the present invention, the above object is achieved through provision of a filter switching circuit including a first amplifier formed on a printed circuit board, a second amplifier formed on the printed circuit board separately from the first amplifier, the second amplifier having an input end opposed to the output end of the first amplifier, a plurality of filters having different frequency passbands, and a plurality of relays each having two fixed terminals and a switching terminal for connecting to one of the fixed terminals, the relays connecting one of the filters between the output end and the input end. The filters and the relays are arranged in parallel in columns in an area between the first amplifier and the second amplifier on the printed circuit board so as to cross a straight line connecting the output end and the input end so that one relay among the relays is positioned almost on the straight line while the switching terminal of the one relay is connected to the output end or the input end, and the other relays are connected in cascade to one fixed terminal of the one relay by connecting each switching terminal thereof to either fixed terminal while connecting each of the filters to the other fixed terminal, whereby the frequency passbands of filters having closer connections to the one relay are sequentially higher increased.
According to another aspect of the present invention, the above object is achieved through provision of a filter switching circuit including a first amplifier formed on a printed circuit board, a second amplifier formed on said printed circuit board separately from said first amplifier, said second amplifier having an input end opposed to the output end of said first amplifier, a plurality of filters having different frequency passbands, and first and second groups of relays each having two fixed terminals and a switching terminal for connecting to one of the fixed terminals, the groups of relays being each formed by a plurality of relays for connecting one of said plurality of relays between the output end and the input end. The relays of the first and second groups and the filters are arranged in parallel in an area between said first amplifier and said second amplifier on said printed circuit board so as to cross a straight line connecting the output end and the input end so that the filters are positioned between the first group of relays and the second group of relays, and one relay in the first group of relays and one relay in the second group of relays are arranged almost on said straight line while the switching terminals thereof are separately connected to the output end and the input end so that the other relays in each group of relays are connected in cascade to one fixed terminal of said one relay in each group of relays by connecting each switching terminal thereof to either fixed terminal, and between the other fixed terminals of two corresponding relays among the other relays in the groups of relays, each of the filters is connected, whereby the frequency passbands of filters having closer connections to the said one relay are sequentially higher increased.
As described above, filters and relays are arranged in parallel in columns in an area between a first amplifier and a second amplifier on a printed circuit board so as to cross a straight line connecting the output end of the first amplifier and the input end of the second amplifier so that one relay among the relays is positioned almost on a straight line connecting the output end and the input end while the switching terminal of the one relay is connected to the output end or the input end, and the other relays are connected in cascade to one fixed terminal of the one relay by connecting each switching terminal thereof to either fixed terminal while connecting each of the filters to the other fixed terminal, whereby the frequency passbands of filters having closer connections to the one relay are sequentially higher increased. Thus, a filter having a higher frequency playback is selected by a smaller number of relays to be connected between the output end and the input end in the smallest distance, and a space on the printed circuit board can be effectively used.
In addition, relays of first and second groups and filters are arranged in parallel in an area between a first amplifier and a second amplifier on a printed circuit board so as to cross a straight line connecting the output end of the first amplifier and the input end of the second amplifier so that the filters are positioned between the first group of relays and the second group of relays, and one relay in the first group of relays and one relay in the second group of relays are arranged almost on the straight line while the switching terminals thereof are separately connected to the output end and the input end so that the other relays in each group of relays are connected in cascade to one fixed terminal of the one relay in each group of relays by connecting each switching terminal thereof to either fixed terminal, and between the other fixed terminals of two corresponding relays among the other relays in the groups of relays, each of the filters is connected, whereby the frequency passbands of filters having closer connections to the one relay are sequentially higher increased. Thus, the selected filter is separated from the other relays, and the characteristics of the filters do not deteriorate.